One method for forming shapes of ceramic materials which has been widely practiced involves extruding a stiff plastic mix through a die orifice. This technique has been commonly used in forming brick, dinnerware, sewer pipe, hollow tile, electrical insulators, and other articles having an axis normal to a fixed cross section. Most generally, a two-stage vacuum de-airing auger has been utilized to remove air bubbles. The bath is thoroughly mixed with water and forced through a die.
More recently, ceramic honeycomb-shaped products, which are composed of a multitude of cells or passages separated by thin walls running parallel to the longitudinal axis of the structure, have been formed through extrusion. Such articles have seen extensive service as filters for fluids, both liquids and gases, and as heat exchangers. Within the past decade the walls of those structures have been coated with a catalyst capable of converting the noxious fumes from the discharge gases of internal combustion engines and wood and coal burning stoves into harmless emissions. It is apparent that the environments in those latter applications require that the structures satisfy a stringent matrix of chemical and physical properties. To illustrate, the mechanical strength thereof must be sufficiently great to resist the mechanical forces necessarily inherent in positioning and mounting the structure in the assembly and the vibrations, gas pressures, and other physical abuse experienced in use. The structures must also manifest high refractoriness, high thermal shock resistace, high resistance to abrasion from particles in the gaseous fumes, and high resistance to chemical attack from the fumes.
Various materials have been investigated for their utility as cellular substrates for catalyst-coated honeycomb structures fashioned via extrusion. That experimentation has included bodies prepared from such materials as alumina-silica, alumina, zirconia-alumina, zirconia-magnesia, mullite, zircon, zircon-mullite, titania, spinel, zirconia, Si.sub.3 N.sub.4, and carbon. Of all the compositions tested, however, only two have been used commercially to any substantial extent, viz., cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) and beta-spodumene solid solution (Li.sub.2 O.Al.sub.2 O.sub.3.2-8SiO.sub.2). And, because the use temperature of beta-spodumene is so low (&lt;1200.degree. C.), its utility for this application is quite restricted. Therefore, cordierite, occasionally combined with a compatible refractory phase, such as mullite (3Al.sub.2 O.sub.3.2SiO.sub.2), has comprised the principal material for extrusion as cellular substrates in catalytic converter assemblies designed for use with wood and coal burning stoves and internal combustion engines.
The present invention is concerned with the extrusion of a plastic ceramic batch into articles of widely-differing profiles and shapes such as, for example, dinnerware and electrical insulators, and especially with the extrusion of thin-walled honeycomb structures from ceramic batches capable of flowing or plastically deforming under pressure during extrusion, but which have the ability to maintain their as-extruded form under ambient conditions after being relieved of the high extrusion shear forces. More specifically, the present invention relates to an apparatus and an improved method for homogenizing ceramic batch materials and extruding them into bodies of various geometries and particularly into cellular substrates.
A typical method currently being utilized for extruding cellular substrates has contemplated the following overall steps:
(1) the batch components are dry mixed together;
(2) water is added and the wet batch thoroughly mixed;
(3) the wet batch is fed to a two-stage, single screw auger system, wherein the first stage compacts and forces the batch through a chip or spaghetti die into a vacuum de-airing chamber, and the second compacts the chips or spaghetti material into billets; and
(4) the billets are ram pressed through a die having the proper configuration to yield honeycomb substrates of a desired structure.
Customarily, a screen will be placed in front of the die to remove any large tramp particles that could injure the die walls or block the orifices in the die.
In general, the batch will comprise a major portion of finely-divided ceramic material, e.g., cordierite with or without mullite, about 1-7%, preferably about 2-5%, of an organic binder/plasticizer, and, optionally, up to about 1% of an extrusion aid. The organic binder/plasticizer exerts a profound effect upon the extrusion properties of the batch, the rate of throughput of material passing the die, and the wet or green strength of the extruded article. The plasticity developed in the batch is strongly dependent upon the rheological characteristics of the binder/plasticizer. Plasticity has been generally defined as that property which enables a material to be deformed without rupture during the application of a stress that exceeds the yield value of the material.
As the ceramic batch is mixed, it develops the plasticity which permits it to be extruded and to retain its geometry after extrusion. The temperature of the batch increases during mixing (this is especially true where shear mixing is employed) because of the energy put into the system. Many organic binders/plasticizers for use with ceramic batches have been investigated including alginates, polyethylene oxides, resins, starches, and waxes. Experience has shown that, commonly, such materials have resulted in the batch softening continuously (the viscosity decreases) as the temperature rises. That phenomenon yields lower extrusion pressures but also, disadvantageously, leads to a loss of shape and reduction of green strength in the extruded substrate.
With certain cellulosic binders there is a slight softening of the batch as the temperature increases, but at a certain characteristic point the system begins to gel, thereby effecting a sharp rise in viscosity. Gelation of the batch can impart improved wet strength to the extruded batch and enhanced retention of strength. The principal, commercially-marketed water soluble cellulose compound utilized for cellular ceramics that demonstrates this desirable behavior is methyl cellulose.
When dissolved in water and heated, that methyl ether polymer forms a gel, but returns to a liquid solution when cooled. A typical viscosity-temperature curve for an aqueous methyl cellulose solution is illustrated in FIG. 1, which reports the gelation of a 2% aqueous solution of a methyl cellulose having a nominal viscosity of 100 mPa.s when heated at 0.25.degree. C./minute. The rate of shear is 86s.sup.-1.
As can be observed in FIG. 1, the viscosity of the solution falls as the temperature is raised until the incipient gelation temperature is attained, at which point there is a dramatic increase in viscosity upon further heating. During cooling, the gel reverts to a liquid and exhibits the properties of the original solution.
FIG. 2 illustrates the typical behavior of ceramic batches containing methyl cellulose as the binder/plasticizer expressed in terms of apparent viscosity as a function of temperature. As the temperature of the gelled ceramic batch is raised, for example, during shear mixing, a point is reached whereat the batch loses its plasticity and becomes separated into discrete granular particles. At that point the ceramic batch can no longer be extruded into a cellular substrate. The temperature of that point controls the rate at which the batch can be extruded and, consequently, the throughput of the extrusion. In general, the higher the temperature that can be utilized, the more rapid will be the rate of extrusion.
Because the cell walls of the desired honeycomb substrate are very thin and, hence, readily susceptible to distortion, the batch therefor is kept at a high viscosity. As currently practiced, the wet batch is commonly mixed in a muller-type mixer and, because the time for mixing is held as brief as possible commensurate with good mixing, little work is done on the batch. FIG. 3 is a diagrammatic sketch portraying the viscosity curve as a function of work with time as measured on a plastograph. As can be seen there, the viscosity of the batch initially rises sharply as work is applied thereto, but it passes through a maximum and levels out to a generally equilibrium viscosity, the temperature of the mix being maintained relatively constant.
For the cordierite and cordierite-mullite batches currently employed for the extrusion of cellular substrates, and wherein methyl cellulose comprises the organic binder/plasticizer, a mulling time of only a few minutes has been found satisfactory to insure good mixing. The narrow interval AB represents the viscosity of the batch following that mixing. Maintenance of that limited window of operable viscosities on a rapidly changing portion of the viscosity curve is, however, affected by several variables; e.g., batch temperature, water temperature, water content, time after mulling, augering (extrusion) of billets, and final extrusion. Also, the viscosity of the mixed batch can change with time as the methyl cellulose continues to hydrate with time. Accordingly, the level of work on the batch is non-uniform throughout the process. Even more critically, the viscosity of the batch can vary across the diameter of an extruded billet.
One method for solving this problem is to mix or work the batch for a sufficient length of time such that the viscosity reaches the relatively flat portion of the curve, i.e., the viscosity approaches an equilibrium, through much extended mixing times. However, to achieve the necessary batch viscosity for extruding the desired cellular substrates would require a batch exhibiting a viscosity curve as described in FIG. 4. Thus, as is depicted in the new curve, the overall viscosity range of the batch must be moved upward (more viscous), such that the equilibrium viscosity is that desired for extrusion.
It is apparent that increased viscosity of the batch can be secured by reducing the water content and/or raising the temperature of the batch. The latter alternative, however, hazards the loss of plasticity of the batch. The former option demands extended mixing times which are unattractive from a practical point of view.
Therefore, a primary objective of this invention is to devise means for putting a great amount of work into the batch in a short period of time and in a uniform manner. Such would exert the same effect as an extended mixing time in a low shear mixer.
A second objective is to devise an organic binder/plasticizer exhibiting the gelation capability of methyl cellulose but operable at higher temperatures.
As has been explained above, billets of the ceramic batch are currently extruded utilizing a single-screw, two-stage auger system. In the first stage the auger picks up the batch and compacts it against an element having apertures therein. As the batch is forced through those holes into a vacuum chamber, it is transformed into chips or into the form of noodles or spaghetti. The compacted material provides a vacuum seal so that air can be readily removed from the batch in the attenuated or shredded state. In the second stage the auger picks up the de-aired material, compacts it into a transition zone to make a billet larger than the auger chamber, and then forces it against a die for extrusion into a billet.
In the current process for forming cellular substrates, the billet, as prepared above, is ram pressed through a die having the proper configuration to yield the desired cellular product. A defect, termed auger spot, has been observed in the center portion of the cellular substrates which has been traced to inhomogeneity of the billet from the auger.
The primary function of the auger is to de-air and compact the batch material and apply pressure thereto. For the most part the material travels through the barrel of the extrusion apparatus as a plug. The material in actual contact with the core and flights of the screw experiences considerable shear when compared to the bulk of the material. And to a lesser degree, the material in contact with the wall of the barrel is subject to more shear than the bulk. As a result, those regions evidence a decrease in viscosity or stiffness because of the added work being applied thereto (FIG. 3). As a consequence, when the material moves beyond the tip of the screw, the material in actual contact with and closely adjacent to the screw collapses to the center of the billet. That phenomenon creates a soft spot in the billet which, when the billet is extruded through the honeycomb die, flows more easily through the die. That action results in the webs in the center of the extruded product being thicker-walled and, in some instances, the channels have been completely filled. The swollen webs impart a shadow effect which is readiy visible in the extruded body. As can easily be appreciated, careful control of auger spot, i.e., inhomogeneous stiffness across the billet, must be exercised in order to prevent loss of ware because of excessively swollen webs in the extruded substrate.
This auger spot must be distinguished from the discoloration occasionally observed which is the result of metal wear of the auger screw. The metallic particles by themselves appear to exert no adverse effect upon the final product.